The shift in military posture would allow North Korea to carry out pre-emptive attacks, and deprive "the enemies to sleep in peace till the moment they meet their final end in their land, KCNA reported, citing a speech by the country's leader, Kim Jong-un.

KCNA continued: "The only way for defending the sovereignty of our nation and its right to existence under the present extreme situation is to bolster up nuclear force both in quality and quantity and keep balance of forces, he said, stressing the need to get the nuclear warheads deployed for national defense always on standby so as to be fired any moment."

The move, announced in an article about missile tests, follows the U.N. Security Council's unanimous approval Wednesday of tough new sanctions against North Korea in response to its recent nuclear and long-range missile tests. The resolution contains the toughest set of sanctions imposed by the Security Council in more than two decades, Secretary of State John Kerry said.

The sanctions require North Korean cargo ships and aircraft to be inspected before entering and after leaving the reclusive country. They would also prohibit small arms and other conventional weapons sales to North Korea.

The Pentagon said it is aware of the reports and closely monitoring the situation in coordination with regional allies.

Gabrielle Price, the spokeswoman for the State Department's Bureau of East Asian and Pacific Affairs, said: "We urge North Korea to refrain from provocative actions and rhetoric that aggravate tensions and instead focus on fulfilling its international obligations and commitments."